They live on plant nectar when not producing eggs
All adult mosquitoes feed on the nectar or honey dew of plants to get sugar, and that provides enough nourishment for both males and females to live, but females also need to produce eggs. To create eggs, females need protein, which they get from the blood of animals. Only a few species of mosquitoes can store enough energy as larvae, to produce eggs when they're adults, without needing a meal of blood.
The blood can come from humans, mammals, birds, reptiles or frogs, though most mosquitoes have a preference for a few particular sources. Some live near humans and bite them, while others choose livestock or animals first.
After the female mates, has her first meal of blood and lays her eggs, she begins looking for another drink of blood, to develop another batch of eggs without mating again. If she survives, she may do this several times in one summer.
When they're not looking for blood, both the males and females feed on a wide variety of plants, though each species has its preferences. "Early spring Aedes mosquitoes have been recorded nectar-feeding on Canada plum, pin cherry and a variety of woodland plants. Even in the high Arctic, mosquitoes have been observed feeding on floral nectar from the flower Dryas integrifolia, [a small white Arctic flower]" according to Encyclopedia of Entomology by John L. Capinera. It's a good reference book to find in the library, but probably not one you'd want to buy!
The same author describes how mosquitoes digest the nectar. After they draw it in through their long proboscis, they store it in a sack-like crop connected to the fore-gut. The fore-gut is divided into two parts, a front section that digests sugars and a rear section that digests blood. The front mid-gut secretes enzymes which digest the nectar into a liquid and it passes from the crop into the front mid-gut.
When the female mosquito feeds on blood, the blood goes directly to the rear midgut, which can digest the protein. The rear midgut can also expand as the mosquito draws in the blood, so she can hold a full meal at once.
A Change of Diet
In some species of mosquitoes, the fertilized females hibernate over the winter, while the males die off when cold weather comes. To survive without food while they're hibernating, the females need to eat extra sugars to double their weight in the fall, so they don't need to feed again until spring. Late in the season, they stop looking for animals to bite and instead seek rotting fruit or nectar.
Two entomologists recently discovered what makes these mosquitoes change their diet in time to fatten up for winter. It all depends on day length. A news release about the discovery explains how it occurs: "As the days begin to get shorter, two genes that code for digesting blood switch off, and a different gene for digesting sugar and retaining fat switches on."
Female mosquitoes have a long, thin proboscis, or mouthpiece, that is similar to male mosquitoes', but more efficient for extracting blood. After landing on a potential victim, she probes the skin and injects saliva. The saliva has chemicals in it which thin the blood, making it easier for her to suck up, and which numb the skin, making the victim less apt to notice what's happening, and which lubricate the area so she can insert the proboscis. The lingering effects of the saliva are also what make mosquito bites itch afterwards.
proboscis's sharp tip.
If she thinks she's found a blood-filled capillary close to the surface, she inserts her proboscis and draws up the blood until her gut is full, or until she's frightened away.
Mosquitoes don't spread disease by transferring blood from one victim to another. Instead, the disease is passed in the injected saliva. That's why only certain diseases can be spread, such as malaria or yellow fever, since they have evolved to live inside the mosquito and spread in her saliva.
Mosquitoes also eat before they become adults. The larvae live in water and feed on microscopic organic particles which they filter from the water through brush-like structures around their mouth. The larvae of a few larger species also eat other small insects, including smaller mosquito larvae.
Photos of mosquitoes feeding courtesy of the CDC. Mosquito proboscis photo from Wikimedia Commons user Ben133uk